The present invention generally relates to automatic white balance adjusting circuits for image pickup devices, and more particularly to an automatic white balance adjusting circuit for an image pickup device, in which the white balance is automatically adjusted by varying amplifications of amplifiers with respect to primary color signals which are obtained by picking up an image depending on a signal obtained from a color temperature detector.
A conventional automatic white balance adjusting circuit for a color image pickup device has the following circuit construction. That is, an image pickup signal which is obtained by picking up an object by a color video camera is separated into a luminance signal Y and three primary color signals of green (G), red (R) and blue (B) in a matrix circuit. The luminance signal Y and the three primary color signals G, R and B are independently amplified in respective voltage amplifiers and are supplied to an encoder which produces from these signals a color video signal in conformance with the NTSC system, for example. This output color video signal of the encoder is outputted via an output terminal.
On the other hand, an output detection voltage of a color temperature detector is supplied to the voltage amplifier for the primary color signal R via a non-inverting amplifier to vary an amplification thereof on one hand, and is supplied to the voltage amplifier for the primary color signal B via an inverting amplifier to vary an amplification thereof on the other. The amplification of the voltage amplifier for the primary color signal R is varied depending on a control voltage from the non-inverting amplifier having a characteristic in accordance with a characteristic of the output detection voltage of the color temperature detector. The amplification of the voltage amplifier for the primary color signal B is varied depending on a control voltage from the inverting amplifier having a characteristic which is an inverse of the characteristic of the output detection voltage of the color temperature detector. Hence, in the case where the color temperature of the object which is picked up is low, that is, in the case where the energy of the long wavelength wave is large and the energy of the short wavelength wave is small, for example, the amplification of the primary color signal B is made large and the amplification of the primary color signal R is made small so as to automatically adjust the white balance of the color video signal which is outputted via the output terminal.
Generally, when picking up an image outdoors on a clear day, the color temperature setting of the color video camera must be fixed to approximately 5500.degree. to 6500.degree. K. However, when picking up an image against the sunlight, although the object has a color temperature of approximately 6500.degree. K. and thousands of lux, the sunlight having a color temperature of approximately 5500.degree. K. and tens of thousands of lux enters a sensor of the color temperature detector. In this case, according to the color video signal which is obtained from the conventional automatic white balance adjusting circuit, the face of the person who is picked up by the image pickup device will look pale. On the other hand, when picking up an image from a low angle, the sensor of the color temperature detector will detect the color temperature of the blue sky and the face of the person will look reddish. Furthermore, in the case where the person who is to be picked up by the image pickup device wears a blue suit and the illumination is a reddish light having a color temperature of 3000.degree. K., the face of the person will look reddish, and there is a disadvantage in that the conventional automatic white balance adjusting circuit cannot perform a satisfactory adjustment of the white balance.
Accordingly, the present inventors have previously proposed an automatic white balance adjusting circuit in a Japanese Patent Application No. 60-46273 filed Mar. 8, 1985. This previously proposed automatic white balance adjusting circuit comprises a color temperature detecting circuit for producing a detection signal having a value in correspondence with a color temperature of an incoming light to the image pickup device, a comparing circuit for obtaining a discrimination signal by comparing the detection signal with a reference D.C. voltage, and a selecting circuit for selecting and obtaining a color temperature setting signal having a value in correspondence with the discrimination signal. The white balance is automatically adjusted by varying the amplifications with respect to the primary color signals depending on the value of the color temperature setting signal.
According to the previously proposed automatic white balance adjusting circuit, the color reproducibility of the picture which is obtained by picking up an image under illuminations having color temperatures of 3200.degree. K. and 5600.degree. K. is stable and satisfactory. However, there is a problem in that the color reproducibility is poor when the image is picked up under an illumination of a fluorescent light such as a white light having a color temperature of 3800.degree. K. an a daylight (white light) having a color temperature of 5000.degree. K., and a mixed light source comprising an incandescent lamp and a fluorescent lamp, for example. In other words, the color reproducibility is poor when the image is picked up under an illumination of a light source having an intermediate color temperature between 3200.degree. K. and 5600.degree. K.